I have a newly-installed, 20A/120V receptacle that is wired using 12/2 copper on a single home run back to my main electrical panel. This was to support an air conditioning unit in a section of new construction on an existing house. Unfortunately the air conditioner actually needs a 20A/240V receptacle.
My question is, can I simply replace the single-pole breaker with a 2-pole breaker, and the receptacle with the correct style (NEMA 6-20R)?
(Clarification: there is a separate 15A branch circuit to satisfy all code requirements for normal receptacles and one of those is on this wall, therefore changing this would not leave me without the required amount of normal receptacles)